The present invention is directed to a process for the manufacture of spiro pentaerythritol diphosphites. More specifically, the present invention is directed to a process for the manufacture of pentaerythritol diphosphites via transesterification under vacuum to produce a diphosphite with high spiro isomer content.
Pentaerythritol diphosphites comprise at least two isomeric forms, the spiro and caged isomers. The commercial desirability of the spiro isomer over the caged isomer is well known. It is, therefore, desirable to produce pentaerythritol diphosphites having high spiro isomer content.
Generally, pentaerythritol diphosphites can be prepared by at least two different methods. In one method, two sequential transesterification reactions are performed, first, the reaction of pentaerythritol with triphenyl phosphite to make diphenyl pentaerythritol diphosphite, and second, the reaction of the intermediate diphenyl pentaerythritol diphosphite with the appropriate alkylphenol or alcohol to produce the desired pentaerythritol diphosphite. In another method, dichloropentaerythritol diphosphite is reacted with the appropriate alkylphenol or alcohol to produce the desired pentaerythritol diphosphite. The latter method is said to produce a bis(alkylphenyl) pentaerythritol diphosphite with a negligible portion of caged isomer but involves more complex and expensive processing technology. The former transesterification method is cheaper to implement but generally produces mixtures of isomers having a spiro isomer content of from 50 percent upon to 75 percent depending on the method of preparation and the reactants. Thus bis(alkylphenyl) pentaerythritol diphosphite prepared via transesterification typically are mixtures of spiro and caged isomers that must be further purified by selective crystallization in order to produce a predominantly spiro product. Such an approach, however, inherently leads to a relatively low yield.
One approach to improving the spiro isomer content of bis (2,4-di-t-butylphenyl) pentaerythritol diphosphite produced via transesterification is to react diphenyl pentaerythritol diphosphite (DPPEDP) with 2,4-di-t-butylphenol in a C10-C16 n-alkane or cycloalkane solvent. This method gives a product with a spiro isomer content of up to 90 percent, but the diphosphite yield is relatively low at about 77 percent.
An economically more favorable and thus desirable transesterification process would be one which produces a bis(alkylphenyl) pentaerythritol diphosphite with a spiro isomer content of at least 90 percent, and with a high diphosphite yield of at least 95 percent.